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Chapter 1
What are We?
How Do We Know?
Guidepost
As you study astronomy, you will learn about yourself.
You are a planet walker, and you should understand
what it means to live on a planet that whirls around a
star drifting through a universe of stars and galaxies.
You owe it to yourself to know where you are. That is
the first step to knowing what you are.
In this chapter, you will meet four essential questions
about astronomy:
• Why should we study astronomy?
• How do scientists know about nature?
• Where are you in the universe?
• How does human history fit on the time scale of the
universe?
Guidepost (continued)
Besides learning about astronomy, you will consider
important questions about science:
• How do we know? How does science work?
• How do we know? What is the difference between a
scientific argument and an advertisement?
In this chapter, a cosmic zoom takes you roaring
outward through the universe checking out its major
features. In the next chapter, you will return to Earth
and begin your study by looking at the stars in the
night sky.
Scales of Size and Time
Astronomy deals with objects on a vast
range of size scales and time scales.
Most of these size and time scales are way
beyond our every-day experience.
Humans, the Earth, and even the solar system
are tiny and unimportant on cosmic scales.
A Campus Scene
16 x 16 m
A City View
1 mile x 1 mile
The Landscape of Pennsylvania
100 miles x 100 miles
The Earth
Diameter of the Earth: 12,756 km
Earth and Moon
Distance Earth – Moon: 384,000 km
Earth Orbiting Around the Sun
Distance Sun – Earth = 150,000,000 km
Earth Orbiting Around the Sun (2)
In order to avoid large numbers beyond our
imagination, we introduce new units:
1 Astronomical Unit (AU)
= Distance Sun – Earth =
150 million km
The Solar System
Approx. 100 AU
(Almost) Empty Space Around
Our Solar System
Approx. 10,000 AU
The Solar Neighborhood
Approx. 17 light years
The Solar Neighborhood (2)
New distance scale:
1 light year (ly) =
Distance traveled by light
in 1 year
= 63,000 AU = 1013 km
= 10,000,000,000,000 km
(= 1 + 13 zeros)
= 10 trillion km
Nearest star to the Sun:
Approx. 17 light years
Proxima Centauri, at a
distance of 4.2 light years
The Extended Solar Neighborhood
Approx. 1,700 light years
The Milky Way Galaxy
Diameter of the Milky Way: ~ 75,000 ly
The Local Group:
Our Cluster of Galaxies
Distance to the nearest large galaxies:
several million light years
The Universe on Very Large Scales
Clusters of galaxies are grouped into superclusters.
Superclusters form filaments and walls around voids.
How Do We Know?
The Scientific Method
Observed facts - phenomenology
Possible explanation – hypothesis / theory
Testable predictions:
What else should we see if the
hypothesis / theory is true?
Further observations to test predictions
Observations contradict
predictions
Modify or reject
hypothesis
Observations confirm
predictions
Support for
hypothesis/theory –
Develop further predictions